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2 This lecture will help you understand:ConductionConvectionRadiationNewton’s Law of CoolingGlobal Warming and Greenhouse Effect

3 Heat Transfer and Change of PhaseObjects in thermal contact at different temperatures tend to reach a common temperature in three ways:ConductionConvectionRadiation

4 Conduction ConductionTransfer of internal energy by electron and molecular collisions within a substance, especially a solid

5 Conduction Conductors Good conductors conduct heat quickly.Substances with loosely held electrons transfer energy quickly to other electrons throughout the solid.Example: Silver, copper, and other solid metals

6 Conduction Conductors (continued) Poor conductors are insulators.molecules with tightly held electrons in a substance vibrate in place and transfer energy slowly—these are good insulators (and poor conductors).Example: Glass, wool, wood, paper, cork, plastic foam, airSubstances that trap air are good insulators.Example: Wool, fur, feathers, and snow

7 A. Yes In some cases, yes No In some cases, no ConductionCHECK YOUR NEIGHBORIf you hold one end of a metal bar against a piece of ice, the end in your hand will soon become cold. Does cold flow from the ice to your hand?A. YesIn some cases, yesNoIn some cases, noC. No.

8 Conduction CHECK YOUR ANSWERIf you hold one end of a metal bar against a piece of ice, the end in your hand will soon become cold. Does cold flow from the ice to your hand?A. YesIn some cases, yesNoIn some cases, noExplanation:Cold does not flow from the ice to your hand. Heat flows from your hand to the ice. The metal is cold to your touch because you are transferring heat to the metal.C. No.

9 Conduction Insulation (continued)Dramatic example: Walking barefoot without burning feet on red-hot coals is due to poor conduction between coals and feet.

10 Convection ConvectionTransfer of heat involving only bulk motion of fluidsExample:Visible shimmer of air above a hot stove or above asphalt on a hot dayVisible shimmers in water due to temperature difference

11 Convection Reason warm air risesWarm air expands, becomes less dense, and is buoyed upward.It rises until its density equals that of the surrounding air.Example: Smoke from a fire rises and blends with the surrounding cool air.

12 Convection Cooling by expansionOpposite to the warming that occurs when air is compressedExample: The “cloudy” region above hot steam issuing from the nozzle of a pressure cooker is cool to the touch (a combination of air expansion and mixing with cooler surrounding air).Careful, the part at the nozzle that you can’t see is steam—ouch!

13 Convection CHECK YOUR NEIGHBORAlthough warm air rises, why are mountaintops cold and snow covered, while the valleys below are relatively warm and green?A. Warm air cools when rising.There is a thick insulating blanket of air above valleys.Both A and B.None of the above.C. Both of the above.

14 Convection CHECK YOUR ANSWERAlthough warm air rises, why are mountaintops cold and snow covered, while the valleys below are relatively warm and green?A. Warm air cools when rising.There is a thick insulating blanket of air above valleys.Both A and B.None of the above.Explanation:Earth’s atmosphere acts as a blanket, which keeps the valleys from freezing at nighttime.C. Both of the above.

15 Convection Winds Result of uneven heating of the air near the groundAbsorption of Sun’s energy occurs more readily on different parts of Earth’s surface.Sea breezeThe ground warms more than water in the daytime.Warm air close to the ground rises and is replaced by cooler air from above the water.